Secondary lithium-ion electrochemical cells typically include a positive electrode that contains lithium in the form of a lithium transition metal oxide, a negative electrode, a separator, and an electrolyte. Examples of transition metal oxides that have been used for positive electrodes include lithium cobalt dioxide and lithium nickel dioxide. Other exemplary lithium transition metal oxide materials that have been used for positive electrodes include mixtures of cobalt, nickel, and/or manganese oxides. Negative electrodes typically include graphite, lithium titanates, or alloys comprising electrochemically active elements such as Si, Sn, Al, Ga, Ge, In, Bi, Pb, Zn, Cd, Hg, and Sb.
The challenges in designing lithium-ion electrochemical cells include obtaining a balance between high capacity, high charge-discharge rates, low irreversible capacity, cost, and safety. Lithium cobalt oxide (LiCoO2) is widely used as the positive electrode in lithium-ion electrochemical cells for use in commercial products such as computers and hand held phones. LiCoO2 electrodes have high capacity due to the high density of LiCoO2, rapid charge-discharge due to its layered structure, and low irreversible capacity. However, LiCoO2 is expensive and subject to occasional runaway thermal reactions. To temper the expense and safety performance, manganese and nickel can be added to the oxide structure forming the so-called NMC (nickel, manganese, cobalt) oxides of lower cost and higher stability. However, the capacity of these oxides has not increased substantially over the capacity of LiCoO2.
Typically, layered lithium transition metal oxides are never fully delithiated during cycling (charging) due to structural instability of the fully delithiated state. Thus, one way to achieve higher capacities is to design positive electrode materials that have increased stability at higher delithiation and, therefore, can be cycled to higher voltages (e.g., 4.3 to 4.8 V vs. Li/Li+ or greater). Typical lithium mixed metal oxide (NMC) positive electrode materials, such as Li[Ni0.42Mn0.42Co0.16]O2, will cycle well up to a voltage charge of about 4.35 V to 4.4 V (vs. Li/Li+). However, when such materials are charged beyond this value, e.g., to 4.8 V vs. Li/Li+, the cycle life is quite poor.